Cell carriers sell their phones all the time. Look at the carriers page for their phones. You will see a price for the phone with and without a contract. In ATT case, the phone would cost you about $650 without a contract. And yes, they would give you the unlock code for it should you buy the phone "off contract".
However, your main concern is getting a phone that has been blacklisted because its been reported as lost/stolen, which is a highly valid concern because it happens all the time. And carriers don't care about it being sold, who has it, etc. If it's blacklisted then that puppy will never see service again. But that doesn't include phones that have been network unlocked.
What it all comes down to is honesty. If the seller is on the up and up and has a good sell rating then I wouldn't be concerned about it. And lets not forget, this is Amazon yer talking about. Amazon doesn't play games when it comes to scams or fraud. They will prosecute because it's their name on the line here. Something may have been sold by one particular person, but it's still going through Amazon to do it.
Hope this eases your mind a bit.
P:S I have the ATT branded Note. Bought off contract. ATT gave me the code to unlock it.
I live life by three rules. Rule one, Sh*t happens, rule two, sh*t happens on a daily basis, rule three, get used to the first two.